Surveillance systems commonly employ optical video cameras operating in the visible light or near IR frequency range to monitor facilities. Historically, these cameras have transmitted analog video images of an area under surveillance to a security monitoring center for inspection and storage. In many facilities, analog video cameras are being replaced with digital cameras that detect and capture still images of events, such as the appearance of an intruder, a malfunction, or a fire within the area under surveillance. Digital cameras provide several advantages over analog video cameras. For example, digital cameras can be radio linked and battery powered to eliminate the need for the costly fixed infrastructure of video cables and power lines, making surveillance systems cheaper and easier to deploy.
However, digital cameras have limited sensitivity, and are not capable of imaging opaque or concealed items. For example, at a point-of-entry into a facility, such as a government building, school, airport or other structure, traditional analog or digital cameras are not able to identify concealed weapons or other contraband (e.g., explosives). Therefore, as a result of the need for improved surveillance systems, various microwave imaging systems have been developed as alternatives to existing optical systems. Microwave radiation is generally defined as electromagnetic radiation having wavelengths between radio waves and infrared waves. Since microwave radiation is non-ionizing, it poses no known health risks to people at moderate power levels. In addition, over the spectral band of microwave radiation, most dielectric materials, such as clothing, paper, plastic and leather are nearly transparent. Therefore, microwave imaging systems have the ability to penetrate clothing to image items concealed by clothing.
At present, there are several microwave imaging techniques available. For example, one technique uses an array of microwave detectors (hereinafter referred to as “antenna elements”) to capture either passive microwave radiation emitted by a target associated with the person or other object or reflected microwave radiation reflected from the target in response to active microwave illumination of the target. A two-dimensional or three-dimensional image of the person or other object is constructed by scanning the array of antenna elements with respect to the target's position and/or adjusting the frequency (or wavelength) of the microwave radiation being transmitted or detected.
Microwave imaging systems typically include transmit, receive and/or reflect antenna arrays for transmitting, receiving and/or reflecting microwave radiation to/from the object. Such antenna arrays can be constructed using traditional analog phased arrays or binary reflector arrays. In either case, the antenna array typically directs a beam of microwave radiation containing a number of individual microwave rays towards a point or area/volume in 3D space corresponding to a voxel or a plurality of voxels in an image of the object, referred to herein as a target. This is accomplished by programming each of the antenna elements in the array with a respective phase shift that allows the antenna element to modify the phase of a respective one of the microwave rays. The phase shift of each antenna element is selected to cause all of the individual microwave rays from each of the antenna elements to arrive at the target substantially in-phase. Examples of programmable antenna arrays are described in U.S. Pat. No. 7,224,314, entitled “A Device for Reflecting Electromagnetic Radiation,” and U.S. patent application Ser. No. 10/997,583, entitled “Broadband Binary Phased Antenna, filed Nov. 24, 2004.”
Recently, “bi-modal” imaging systems that augment an optical imaging system, such as an analog video camera or digital camera, with a microwave imaging system have been proposed. The optical image captured by the optical imaging system is combined with the microwave image captured by the microwave imaging system to produce a more detailed image than that achievable with either imaging system alone.
In a typical configuration, the optical camera is mounted in the center of the array of antenna elements and a microwave transceiver is mounted on an arm orthogonal to the antenna array to transmit/receive microwave radiation to/from the antenna array. This configuration ensures that the optical camera does not block transmission of the reflected microwave radiation towards the microwave transceiver, and therefore, the optical camera does not interfere with the microwave image. However, in this configuration, the resulting optical and microwave images experience a parallax effect (shifting) due to the different focal points of the optical camera and the microwave transceiver, which increases the complexity of the image processing. In addition, the mounting arms necessarily increase the size of the bi-modal imaging system, which may not be desirable in some situations.
Therefore, what is needed is a bi-modal imaging system with reduced image processing. In addition, what is needed is a compact bi-modal imaging system.